Underground oil extraction tools work in wells thousands of meters deep, with harsh environments and complex stress conditions. Under normal circumstances, the extraction tools must not only withstand tensile stress, and torsional bending stress but also withstand strong friction and impact. At the same time, the tools must also withstand high temperatures, high pressure, and environmental corrosion.

This requires that the material properties of underground mining tools have excellent comprehensive mechanical properties, not only to ensure high strength, but also to ensure excellent impact toughness, and at the same time to resist corrosion from seawater and mud. In view of the performance requirements of underground working conditions, the material selection of underground tools is usually alloy structural steel containing corrosion-resistant elements such as Cr and Mo, and then appropriate heat treatment quenching and tempering processes are used to ensure that it meets the strength and impact toughness requirements. This article focuses on the quenching and tempering of one of the axial pipe workpieces made of 40CrMnMo steel during the processing of underground pipe strings. During the quenching process, severe cracking occurred many times, resulting in the scrapping of the workpiece and causing certain economic losses. To this end, the causes of quenching cracks were analyzed from the aspects of the chemical composition, organization, heat treatment process, and crack morphology of the axis tube material, and improvement and prevention measures were proposed.

1. Description of failed workpiece
The raw material is 40CMnMo steel solid forging material with a size of φ200 mmx1 m. Process flow: rough turning → drilling and boring (to a wall thickness of about 20 mm) → quenching → tempering → fine turning. The outer dimensions of the axis tube workpiece: are a tube with a length of about 1m, a diameter of φ200 mm, and a wall thickness of 20 mm.
Heat treatment process: first slowly heated to 500℃ in a box furnace, then placed in a salt bath furnace and heated to a quenching temperature of 860~880℃, the heating time in the salt bath furnace is about 30min, and then quenched in a quenching oil at about 40-60℃, the quenching time is about 10min, after taking it out, tempered in a box furnace, kept at 600℃ for 10h and cooled with the furnace.
Cracking: The crack develops along the axis of the central tube, which is clearly visible from the edge and has cracked through in the radial wall thickness direction.

2. Inspection and analysis
2.1 Chemical composition inspection: The quenched and cracked axis tube workpiece was sampled by local wire cutting for composition analysis. Its chemical composition conforms to GB/T3077--1999 "Chemical Composition and Mechanical Properties of Alloy Structural Steel".
2.2 Metallographic inspection and analysis: Two samples were taken longitudinally from the quenched and tempered axis tube, fire-treated (850℃ for 15 h and cooled with the furnace), then polished on a polishing machine after sandpaper grinding, and corroded with 4% nitric acid alcohol to observe its metallographic structure.

3. Analysis of cracking causes and solutions
3.1 Crack shape and heat treatment process: Observe the shape of the crack of the axis tube, which is a longitudinal crack. It occurs along the axial direction and the crack is deep. It is even obvious that the crack has been cracked through in the radial direction on the edge of the axis tube. It is concluded that the stress that causes the cracking of the axis tube is the surface tangential tensile stress, which is caused by the later organizational stress. At the same time, since the axis tube is made of medium-carbon alloy structural steel, organizational stress is also the dominant factor in the quenching process. When martensitic transformation occurs, the plasticity decreases sharply, and at this time, the organizational stress increases sharply, so that the tensile stress formed by the quenching internal stress on the workpiece surface exceeds the strength of the steel during cooling and causes cracking, which often occurs in all hardened parts. The main reason for this crack is that the organizational stress is large due to improper quenching process. Since the quenching heating temperature of the axis tube is 860~880℃, the temperature is relatively high, and then it is quickly placed in the quenching oil of 40~60℃. When it is above the Ms transformation temperature, due to the high quenching heating temperature. The thermal stress is large, and when cooled below the MS transformation temperature, the quenching oil temperature is relatively low, and the quenching time of 10 minutes is relatively long. In the process of rapid cooling, more martensite is produced. Due to the different specific volumes of different organizations, larger organizational stress is generated, which is one of the reasons for the quenching cracking of the axial tube.
3.2 Uniformity of raw material organization: After annealing the cut sample 1 (850℃ insulation for 15 hours and furnace cooling), metallographic analysis found that after annealing, the axial tube with cracks still has obvious banded organization segregation, indicating that the copper material itself has serious banded organization segregation and uneven organization. The existence of a banded organization will increase the tendency of workpiece quenching cracking. Relevant literature points out that the banded organization in low and medium-carbon alloy steel refers to the cast organization formed along the rolling direction or forging direction of the steel, which is mainly composed of pro eutectoid ferrite and pearlite. It is a defective organization that often appears in steel.
Due to the selective crystallization of molten steel during the ingot crystallization process to form a dendritic structure with uneven chemical composition distribution, the coarse dendrites in the ingot are elongated along the deformation direction during rolling or forging, and gradually become consistent with the deformation direction, thereby forming carbon and alloy element depleted bands (actually strips) and depleted bands that alternately stack with each other. Under slow cooling conditions, eutectoid ferrite is first precipitated in the carbon and alloy element depleted bands (supercooled austenite has low stability), and the excess carbon is discharged into the enriched bands on both sides, finally forming a band dominated by ferrite: a carbon and alloy element enriched band, whose supercooled austenite has high stability, and thereafter a band dominated by pearlite is formed, thus forming a banded structure in which bands dominated by ferrite and bands dominated by pearlite alternate with each other. The different microstructures of adjacent bands in the banded structure of the axial tube and the differences in the morphology and level of the banded structure cause the expansion coefficient and the difference in specific volume before and after the phase change of the axial tube to increase during the heat treatment quenching process, thereby generating large organizational stress, and ultimately increasing the quenching distortion of the axial tube. If the quenching process is improper, the tendency of the banded structure to cause quenching distortion and cracking will increase, and it is more likely to cause quenching cracking.
3.3 Solution and effect: Through the above analysis of the reasons for the cracking of the axial tube during quenching, the heat treatment quenching process was first improved, the quenching temperature was reduced by about 10°C, and the quenching oil temperature was increased to about 90°C. At the same time, the time of the axial tube in the quenching oil was shortened. The results show that the axial tube did not crack during quenching. It can be seen that the main cause of the quenching cracking of the axial tube is the improper quenching process, and the banded structure in the raw material will increase the quenching cracking tendency of the axial tube, but it is not the main cause of quenching cracking. The sealing test of the axial tube was carried out, and the pressure could be stabilized for 10 minutes at a pressure of 3500 psi (equivalent to 24 MPa), which fully meets the sealing requirements of downhole tools.

4. Conclusion
The main reason for the quenching cracking of the axial tube is the inappropriate quenching process, and the banded structure in the raw material increases the quenching cracking of the axial tube, but it is not the main cause of the quenching cracking. After improving the heat treatment process, the axial tube was no longer quenched and cracked. When the sealing test of the axial tube was carried out, the pressure could be stabilized for 10 minutes at a pressure of 3500 psi (equivalent to 24 MPa), which fully meets the sealing requirements of downhole tools. To prevent the axial tube from cracking during the quenching process, it is necessary to pay attention to the following:
1) Keep good control of the raw materials. The banded structure in the raw materials is required to be ≤3 Levels, and various defects in the raw materials such as looseness, segregation, non-metallic inclusions, etc. meet the standard requirements, and the chemical composition and microstructure must be uniform.
2) Reduce machining stress. Ensure a reasonable feed rate, reduce machining residual stress, or perform tempering or normalizing before quenching to eliminate machining stress.
3) Choose a reasonable quenching process to reduce organizational stress and thermal stress. Appropriately reduce the quenching heating temperature and increase the quenching oil temperature to about 90°C. At the same time, shorten the residence time of the axis tube in the quenching oil.